The present invention is suitable for use with the scanning electron beam computed tomography scanner disclosed in U.S. Pat. No. 4,521,901 which has been assigned to the assignee of the present application. This patent discloses a scanner including a vacuum chamber, an electron gun for producing an electron beam at one end of the chamber, and means for causing the electron beam to scan along an elongated target spaced from the electron gun located within the other end of the chamber. The electron beam impinges on the target and produces X-rays. This patent also discloses a means for causing the cross section of the electron beam--hereafter referred to as the beam spot--at its point of impingement on the target to be generally elliptical in configuration.
U.S. Pat. No. 4,631,741 discloses a method for monitoring the profile, position and orientation of the beam spot at a plurality of positions along the target to determine if the actual profile, position and orientation of the beam spot are equal to the desired profile, position and orientation of the beam spot at these various positions along the target. This method comprises locating a series of electron beam intercepting devices at various points in front of the target, each of which is designed to produce an electrical signal upon impingement of the beam such that the configuration of this signal varies with the profile, position and orientation of the beam as the beam impinges the device. These signals are processed and directed to an oscilloscope which is viewed by an operator who makes corrections to the electron beam direction and shape in order to maintain the desired beam spot profile, position, and orientation. This method is governed by a "trial and error" approach and requires a highly trained technical expert. As a result, the tuning process is extremely slow. The present application more clearly defines the process of correcting the electron beam in order to maintain the desired beam spot behavior and is performed quickly and can be performed automatically without any input from the user.